The field of this invention is the administration of hormones to pregnant cattle for modification of calving. The method is particularly concerned with the administration of relaxin around the time of parturition.
Parturition can be induced in cattle by treatment with corticoids: dexamethasone, dexamethasone trimethylacetate, flumethasone and betamethasone (Davis et al., J. Anim. Sci., 49:560-566 (1979); Welch et al., New Zeal. Vet. J., (1979), 25:111; Diskin, Vet. Record. (1982) 110:268; with prostaglandins (PGF); tham salt of PGF.sub.2.alpha. or its analogs, cloprostenol and fenprostalene (Lauderdale, Annal. Biol. Anim. Biophys. (1975) 15:415; Johnson, Acta. Vet. Scand. (1981) 77:311; and with estrogens (Henricks et al., J. Anim. Sci., (1977), 44:438. Corticoids and prostaglandins cause dystocia and retained placenta, and estrogen treatment does not reduce the incidence of retained placenta. Oxytocin injections are successful only in those animals that have had sufficient cervical softening (Veznik et al., Am. J. Vet. Res., 40:425 (1979).
The peptide hormone relaxin is biologically active in female mammals. It has been prepared principally by purification from the ovaries of pregnant sows. See, for example, Sherwood et al, Arch. Biochem. Biophys. (1974) 160:185-186. The purified porcine relaxin has been studied and sequenced by Dr. Christian Schwabe and associates: Science (1977) 197:914-915; Biochem. Biophys. Res. Comm. (1976) 70(2):397-405; and Biochem. Biophys. Res. Comm. (1977) (2) 75:503-510.
Relaxin is normally produced in the corpora lutea of mammals during late pregnancy and acts in conjunction with estrogen on the structures of the birth canal. Relaxin acts to promote cervical dilation and widening of the birth canal. Purified porcine relaxin administered to cows a few days before expected parturition does induce cervical dilation as the principal effect. See Perezgrovas and Anderson, Biol. Reprod. (1982) 26:765-776. A similar effect is obtained in pregnant sows. See Kertiles and Anderson, Biol. Reprod. (1979) 21:57-68. Perezgrovas et al. (1982) found that administration of porcine relaxin (3000 U/mg) four days before expected parturition did not significantly reduce the duration of pregnancy. Some shortening of pregnancy was observed with heifers given relaxin intracervically on three consecutive days. The authors observed, however, that the observed "reduction probably is of little biological significance."